Combined Gas Law

I am studying National 5 physics, and my exam is tomorrow. I need some last-minute help.
I'm struggling a little bit with the combined gas law, P1V1/T1 = P2V2/T2, and I'm not sure why.
It would help me a lot if you could show step-by-step how the formula is used.
Thanks for your time, it's very much appreciated.

That's the problem: I'm not sure where things are going wrong. I think it may be when I substitute in the values and then have to rearrange the equation, which seems strange because usually rearranging equations is the easiest part!